LibreOffice can use wildcards, or regular expressions, or none - depending on the settings in the Find & Replace
dialog. These settings are hidden under the Other Options
expanding group. So you need to make sure that you select the correct setting.
As you don’t tell which search string are you using, and which text in a cell you are expecting to find, it’s hard to tell if there is anything else to take into account. Note that for formulas, there’s a setting under Options
→LibreOffice Calc
→Calculate
, which tells if regular expressions or wildcards should be used in searches.